Mobile phones and other electronic devices, for example, game controllers, require a vibration source to signal events without creating audible sound, but that are perceived only through the sense of touch. For example, these events might include an incoming phone call, incoming text message, or turbulence of a virtual aeroplane in a computer video game. The vibrations generated by the vibration source must be sufficiently strong to be felt by a person holding the device. These vibration sources are most generally referred to as vibration motors or haptic actuators. One common type of vibration motor is an electromagnetic motor with a rotating shaft and an unbalanced mass attached to the shaft that generates oscillating centripetal force perpendicular to the axis of rotation. Currently, more than one billion vibration motors are manufactured each year and the typical rotation speed is 100 to 300 Hz and the typical centripetal force is 0.1 to 1 N.
Exemplary electromagnetic rotary vibration motors are produced by many companies including Minebea Motor Manufacturing Company of Tokyo, Japan, Sanyo Seitmitsu Co. Ltd. of Nagano, Japan, and KTOL-Jinlong Machinery & Electronics Co. Ltd. of Zhejiang, China. Some versions are tubular type vibration motors and some are disk type vibration motors. For example some of the smallest tubular motors are about 4 mm in diameter and 6 mm in length with a shaft extending from one end of the motor about 4 mm with an unbalanced tungsten mass mounted on the extended shaft. The smallest disk type motors are 10 mm in diameter and 2 mm thick with the tungsten mass rotating inside the motor housing. For both types of motors, a torque is generated to rotate the shaft using conventional direct current (DC) motor designs that include copper coils, iron cores, permanent magnets and coil switching using brushes and armature. Tungsten is used for the mass because its density is more than twice the density of steel. For a tubular motor, a typical Tungsten mass is 0.4 grams with a center of gravity offset 1 mm from the centerline of shaft rotation creating an unbalance mass. For this example when the mass rotates, for example, at 200 Hz (1,256 Rad/sec) the generated centripetal force Fc=(Mass)×(angular velocity)2×(Radius of Offset), which equals: 0.0004 Kg×(1256 Rad/sec)2×0.001 M, or Fc=0.63 N. This dynamic force is sufficient to accelerate the entire mobile phone handset and create vibrations that are perceived by the user.
Unfortunately, a limitation of existing electromagnetic motors, for example, DC vibration motors, is they produce interfering magnetic fields and are constructed of ferromagnetic and conductive materials. The magnetic interference produced by these motors interferes with the operation of other devices in mobile phones, for example, a compass. This problem is growing as mobile phones add additional devices and also continue to become smaller rated. DC motors also are made from conductive materials that are not transparent to radio frequencies (RF) and can not be located near a radio antenna of a wireless communications device.
Ceramic motors, e.g., piezoelectric ultrasonic motors do not generate magnetic fields, are not made from ferromagnetic materials and can be made substantially from non-conductive materials that are substantially RF transparent. A non-magnetic and RF transparent piezoelectric motor has many exemplary advantages for integration in highly miniaturized mobile phones. These piezoelectric motors that generate rotation and also can be used to generate vibration.
Conventional standing wave tubular ultrasonic motors that produce bi-directional rotation or translation use multiple piezoelectric ceramic elements that are either a single ceramic component that is partitioned electrically into multiple independent segments or separate ceramic components. The piezoelectric elements are electrically driven by independent circuits and produce bidirectional motion using a two-phase drive signal with an adjustable phase or by switching a single phase from one piezoelectric element to another. These piezoelectric motors need one or more contact points between the vibrating tube and the rotating shaft that uses axial preload (parallel to the shaft centerline) to generate the contact friction needed to generate torque on the shaft. Unfortunately, these piezoelectric motors using axial preload to generate friction torque generally produce lower output speed and efficiency.